Tank-venting apparatus having the following features have been known for some time: an adsorption filter having a venting opening at its venting end and an intake end to which a supply line from the tank is connected; a tank-venting valve which is connected into a connecting line between the intake pipe of the engine and the intake end of the adsorption filter; and, a drive unit for the tank-venting valve.
The drive unit controls the tank-venting valve in a fixed pregiven time sequence. For example, the drive unit holds the valve closed each time for 1.5 minutes and then opens the valve each time for 4 minutes in order to make possible a regeneration of the adsorption filter. The opening cross section of the tank-venting valve is then determined over a pulse-duty factor dependent upon the particular operating state. For small loads and especially during idle, the tank-venting valve is driven with such a pulse-duty factor that its mean opening cross section is small; whereas, at full load, the valve is kept completely open during the above-mentioned 4 minutes (the so-called tank-venting phase) with the precondition that full load is present for the entire time duration. The highest throughput of ventilating air is present in an upper load range below full load.
It is apparent that tank-venting apparatus of the above kind can only then operate fully satisfactorily when they are tight and when the tank-venting valve opens and closes properly. For checking the apparatus, a method is known according to which an investigation is made especially at low loads as to whether the lambda controller on an internal combustion engine, which is supplied with the gas from the tank-venting apparatus, must make a lean correction during the tank-venting phase. If this is the case, an indication is provided that fuel vapor was supplied from the apparatus. However, this case applies only when the apparatus is tight and the tank-venting valve opens properly. However, no unequivocal statement is made if no lean correction is determined. It is possible that the fuel in the tank does not release vapor so that no fuel vapor is supplied from the apparatus notwithstanding a completely operational tank-venting apparatus.
In order to obtain more reliable data, the California Environmental Authority (CARB) issued a directive in 1989 according to which a tank-venting apparatus is checked in that a level sensor and a temperature sensor are mounted in the tank and the signals of these sensors are evaluated. If the level sensor indicates that fuel has been tanked and at the same time the temperature sensor indicates that the fuel should vaporize adequately, the check as to the above-mentioned lean correction should be made. However, if fuel is added to the tank which has been fully vaporized, then the fuel temperature must be relatively high if an adequate quantity of fuel is to vaporize for a detectable lean correction. If the temperature at which the lean correction investigation is to be carried out is set so high that the vaporized fuel vaporizes adequately for a reliable measurement result, then no diagnosis is undertaken during cold weather and over many trips even though one diagnosis per trip is desired.